The present invention relates to a blocked developing agent or developing agent precursor which is incorporated in a color photographic material. In particular, it relates to a blocked developing agent which is incorporated in a color photographic material for use with a latent or silver image intensification or amplification method.
Silver halides, due to their favorable photosensitivity and oxidant characteristics, have long been employed in photographic materials to assist in the formation of color dye images. It is desirable, however, to reduce the amount of silver halide necessary to form a certain amount of dye image. Reducing the amount of silver halide reduces the cost of the photographic material, the amount of fix and bleach necessary to process the photographic material and the amount of environmentally harmful waste products. Efforts to reduce the amount of silver halide coated in the photographic material have focused on the so-called image intensification or amplification method, as described, for example, in Japanese Laid-Open Application No. 61/77,851 and U.S. Pat. Nos. 4,526,860, 4,469,780, 4,371,609 and 4,045,225. According to this method, a smaller amount of silver halide than usual is incorporated in the photographic material. This type of photographic material is referred to herein as a "low silver laydown" photographic material. The low silver laydown material is exposed and the silver halide is reduced imagewise to silver metal by a developing agent. An amplifying agent then is brought into contact with a developing agent in the presence of the resulting silver nuclei which act as a catalyst for the oxidation of the developing agent by the amplifying agent. The oxidized developing agent then reacts with couplers to form dye images.
Among known intensifying agents are peroxides, halogenous acids, iodoso compounds and cobalt (III) complexes, of which hydrogen peroxide is said to have higher amplification activity. For example, at page 406 of History of Color Photography by J. S. Friedman, there is described a process of oxidizing a paraphenylenediamine color developing agent with hydrogen peroxide in the presence of a silver catalyst.
The latent or silver image amplification method, however, has not yet been put into practical use because of problems existing with the current technology. One difficulty is that in typical amplifying processes the amplifying agent and the developing agent exist together in a bath. The stability or shelf-life of such a mixture, however, is very limited since the developing agent is oxidized by the hydrogen peroxide. To overcome this problem it has been proposed to incorporate the developing agent in the silver halide-containing photographic material. For example, U.S. Pat. No. 4,157,915 describes incorporating carbamate blocked p-phenylenediamines and U.S. Pat. No. 4,371,609 describes incorporating an aromatic primary amine compound.
Previous efforts at incorporating developing agents in the photographic material, though, have caused severe fogging problems, desensitization of the silver halide emulsion, and raw stock storage instability.
A need exists, therefore, for a color developing agent which can be incorporated into a photographic material for use with a silver image amplification method that does not cause fogging or emulsion desensitization. It would be desirable that the oxidation of the incorporated color developing agent be facilitated so as to form indoaniline dyes imagewise using conventionally incorporated couplers.